As an electromagnetic wave detecting device for detecting an electromagnetic wave input to an optical effect unit by probe light pulses, for example, a device for measuring a terahertz wave temporal waveform by a single probe light pulse has been known (Patent Document 1). Terahertz waves are electromagnetic waves having a frequency of approximately 0.01 THz to 1000 THz corresponding to an intermediate range between light waves and radio waves, and have intermediate characteristics between light waves and radio waves. As application of such a terahertz wave, a technique for obtaining information on an object to be measured by measuring a temporal waveform of an electric field amplitude of a terahertz wave generated in, transmitted through, or reflected on the object to be measured has been studied.
In such an electromagnetic wave detection device, the pulse front of probe light pulses is tilted by a pulse front tilting unit, and then the probe light pulses are input to the optical effect unit in some cases. Also, in some cases, the pulse front of probe light pulses is tilted by a pulse front tilting unit, the beam diameter of the probe light pulses with a tilted pulse front is adjusted by a beam diameter adjusting optical system, and then the probe light pulses are input to the optical effect unit.
The pulse front tilt angle (tilt angle with respect to a plane normal to a principal ray direction) γ of light pulses to be output from the pulse front tilting unit is expressed by formula tan γ=λ·dφ/dλ. λ means the wavelength of light pulses, and dφ/dλ means angular dispersion of the pulse front tilting unit at the wavelength λ.
When the beam diameter of light pulses with a tilted pulse front is enlarged or reduced by the beam diameter adjusting optical system, the pulse front tilt angle of the light pulses changes. At this time, the change in the pulse front tilt angle of the light pulses is according to the ratio of enlargement or reduction in the beam diameter of light pulses by the beam diameter adjusting optical system.